1. Field of the Invention
The present invention relates to an image processing technique for correcting image data read by an image reading apparatus such as an image scanner, a copying machine, or a multifunction peripheral.
2. Description of the Related Art
When a user uses an image reading apparatus such as an image scanner or a copying machine to read or make a copy of a bound document such as a book or a magazine, the user needs to place two facing pages of the bound document face-down on a document positioning glass plate. The document is then illuminated through the document positioning plate, and reflected light from the document is read through the document positioning plate. In this way of reading a document, a bound portion near a boundary between the two facing pages of the document placed face-down and end portions (hereinafter referred to as “edge portions”) where paper sheets overlap one another on the both sides of the document are lifted from the document positioning plate.
As a result, the bound portion and edge portions of the read image data include shadows or distortion, and visibility of an output image is deteriorated. Particularly, if an image includes text in dark areas near the bound portion, the text is hard to recognize and read. This is particularly problematic in the case of thick books. Further, if such image data is printed, toner or ink is unnecessarily consumed, and printing costs per page are increased.
However, to obtain good image quality, if the bound portion of a book document is tightly pressed onto a document positioning plate to prevent the lift of the document from the document positioning plate, the book document may be damaged.
To solve this problem, a method referred to as “shape from shading” is known. The method is used to correct dark areas of a copy of a bound document by estimating three-dimensional (3D) shape of an object, namely, a shape of a lifted document, based on image density distribution information. However, since this method requires an enormous amount of calculation and longtime for correction processing, it is difficult to put the method to practical use.
To effectively correct such dark areas with a less amount of calculation, U.S. Pat. No. 6,330,050 discusses an image correction apparatus which estimates a 3D shape of a book document based on a shape of a page contour line of a read image.
Further, US Patent Application No. 2008/0024845, the assignee of which is the same as that of the present invention, discusses an image reading method for automatically detecting and correcting shadow areas.
However, most image reading apparatuses such as small-sized image scanners, multifunction peripherals, and copying machines are limited in a surface size (A4 size or letter size) of the document positioning plate or a direction in which a cover of the document positioning plate opens. Thus, a user cannot always place a document in a given direction. Namely, based on the above conventional method, all of two facing pages of a document and the contour lines of the pages cannot be read, and thus, suitable correction results may not be obtained.